For more than a decade, the merger approval process has served as a way for the government to boost its oversight over telecom companies. The key: increasingly restrictive security agreements between telecom companies and national security agencies that are worked into deals. The agreements are designed to stave off network breaches and allow the government to monitor communications and access companies’ systems upon request.

Three of the top four U.S. wireless carriers—Sprint, T-Mobile, Verizon and Vodafone—and three major equipment suppliers operate under such agreements.

Experts say the security agreements’ scopes are expanding. "Each agreement seems to become more restrictive as the government recognizes the benefits of access to networks and databases and as threats to national security increase," Warren Lavey, a former Skadden partner who worked on telecom mergers, told the Wall Street Journal.